1. Field of the Invention
Aspects of the present invention relate to a negative active material, a negative electrode including the negative active material, and a rechargeable lithium battery including the negative electrode.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as power sources of portable electronic devices. They use an organic electrolyte solution and have twice the discharge voltage and a higher energy density, than a conventional battery using an alkali aqueous solution. For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides that can intercalate lithium, such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, and the like, have been researched.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions, have been used. Graphite increases the discharge voltage and energy density of a battery, because it has a lower discharge potential of −0.2V, as compared to lithium. A battery using graphite as a negative active material has a high average discharge potential of 3.6V and an excellent energy density. Furthermore, graphite is the most comprehensively used material, among the aforementioned carbon-based materials, since graphite guarantees a better cycle-life for a battery, due to its outstanding reversibility. However, a graphite active material has a low density, and consequently, a low capacity in terms of energy density per unit volume. Further, graphite is likely to react with an organic electrolyte at a high discharge voltage, which can lead to battery combustion or explosion.
In order to solve these problems, a great deal of research on oxide negative active materials has recently been performed. For example, amorphous tin oxide developed by Fuji Film Co., Ltd. Japan has a high capacity per weight (800 mAh/g). However, this oxide has resulted in some critical defects, such as a high initial irreversible capacity of up to 50%. Furthermore, the tin oxide tends to be reduced into tin metal during the charge or discharge reactions, which reduces its acceptance for use in a battery.
Referring to another oxide negative electrode, a negative active material of LiaMgbVOc (0.05≦a≦3, 0.12≦b≦2, 2≦2c-a-2b≦5) is disclosed in Japanese Patent Publication 2002-216753. The characteristics of a rechargeable lithium battery including Li1.1V0.9O2 were also presented in the 2002 Japanese Battery Conference (Preview No. 3B05). However, such an oxide negative electrode does not show sufficient battery performance, and therefore, there is ongoing research into other oxide negative materials.